The present invention relates to very high areal recording density longitudinal magnetic recording media exhibiting improved thermal stability, such as hard disks. More particularly, the present invention relates to improved magnetic recording media including an interface layer between a spacer layer and a magnetic layer for providing enhanced magnetic coupling, i.e., RKKY-type coupling, between spaced-apart ferromagnetic layers.
Magnetic recording (xe2x80x9cMRxe2x80x9d) media and devices incorporating same are widely employed in various applications, particularly in the computer industry for data/information storage and retrieval applications, typically in disk form. Conventional magnetic thin-film media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording medium layer, are generally classified as xe2x80x9clongitudinalxe2x80x9d or xe2x80x9cperpendicularxe2x80x9d, depending upon the orientation of the magnetic domains of the grains of magnetic material.
A conventional longitudinal recording, hard disk-type magnetic recording medium 1 commonly employed in computer-related applications is schematically illustrated in FIG. 1, and comprises a substantially rigid, non-magnetic metal or glass substrate 10, typically of aluminum (Al) or an aluminum-based alloy, such as an aluminum-magnesium (Alxe2x80x94Mg) alloy, having sequentially deposited or otherwise formed on a surface 10A thereof a plating layer 11, such as of amorphous nickel-phosphorus (Nixe2x80x94P); a seed layer 12A of an amorphous or fine-grained material, e.g., a nickel-aluminum (Nixe2x80x94Al) alloy, a chromium-titanium (Crxe2x80x94Ti) alloy, a tantalum (Ta) layer, or a tantalum nitride (TaN) layer; a polycrystalline underlayer 12B, typically of Cr or a Cr-based alloy, a magnetic recording layer 13, e.g., of a cobalt (Co)-based alloy with one or more of platinum (Pt), Cr, boron (B), etc.; a protective overcoat layer 14, typically containing carbon (C), e.g., diamond-like carbon (xe2x80x9cDLCxe2x80x9d); and a lubricant topcoat layer 15, e.g., of a perfluoropolyether. Each of layers 11-14 may be deposited by suitable physical vapor deposition (xe2x80x9cPVDxe2x80x9d) techniques, such as sputtering, and layer 15 is typically deposited by dipping or spraying.
In operation of medium 1, the magnetic layer 13 is locally magnetized by a write transducer, or write xe2x80x9cheadxe2x80x9d, to record and thereby store data/information therein. The write transducer or head creates a highly concentrated magnetic field which alternates direction based on the bits of information to be stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the material of the recording medium layer 13, the grains of the polycrystalline material at that location are magnetized. The grains retain their magnetization after the magnetic field applied thereto by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the recording medium layer 13 can subsequently produce an electrical response in a read transducer, or read xe2x80x9cheadxe2x80x9d, allowing the stored information to be read.
Efforts are continually being made with the aim of increasing the areal recording density, i.e., the bit density, or bits/unit area, and signal-to-medium noise ratio (xe2x80x9cSMNRxe2x80x9d) of the magnetic media. However, severe difficulties are encountered when the bit density of longitudinal media is increased above about 20-50 Gb/in2 in order to form ultra-high recording density media, such as thermal instability, because the necessary reduction in grain size reduces the magnetic energy, Em, of the grains to near the superparamagnetic limit, whereby the grains become thermally unstable. Such thermal instability can, inter alia, cause undesirable decay of the output signal of hard disk drives, and in extreme instances, result in total data loss and collapse of the magnetic bits.
One proposed solution to the problem of thermal instability arising from the very small grain sizes associated with ultra-high recording density magnetic recording media, is to increase the crystalline anisotropy and therefore increase the magnetic energy of the grains, thus the squareness of the magnetic bits, in order to compensate for the smaller grain sizes. However, this approach is limited by the field provided by the writing head.
Another proposed solution to the problem of thermal instability of very fine-grained magnetic recording media is to provide stabilization via coupling of the ferromagnetic recording layer with another ferromagnetic layer or an anti-ferromagnetic layer. In this regard, it has been recently proposed (E. N. Abarra et al., IEEE Conference on Magnetics, Toronto, April 2000) to provide a stabilized magnetic recording medium comprised of at least a pair of ferromagnetic layers which are anti-ferromagnetically-coupled (xe2x80x9cAFCxe2x80x9d) by means of an interposed thin, non-magnetic spacer layer. The coupling is presumed to increase the effective volume of each of the magnetic grains, thereby increasing their stability; the coupling strength between the ferromagnetic layer pairs being a key parameter in determining the increase in stability.
However, a significant drawback associated with the above approach is observed when a pair of ferromagnetic layers of alloy compositions which exhibit superior performance when utilized in conventional longitudinal magnetic recording media, e.g., Coxe2x80x94Cr and Coxe2x80x94Crxe2x80x94Pt alloys, are coupled across an interposed thin, non-magnetic spacer layer. Specifically, the observed coupling is, in general, significantly lower than that observed with layers composed of pure (i.e., unalloyed) Co. For example, FIG. 2 shows M(H) loops in the first quadrant for graphically illustrating the decrease in anti-ferromagnetic coupling (xe2x80x9cAFCxe2x80x9d), i.e., saturation fields, between a pair of Co100xe2x88x92xCrx layers across a ruthenium (Ru) non-magnetic spacer layer (where the Ru layer thickness was 8 xc3x85 for maximizing the value of AFC), as the amount of Co decreases in sandwich-type Co100xe2x88x92xCrx (30 xc3x85)/Ru (8 xc3x85)/Co100xe2x88x92xCrx (30 xc3x85) structures, for x increasing stepwise from 0 to 20 (i.e., x=0, 5, 10, 15, and 20). As may be appreciated, the saturation fields, and therefore the strength of AFC, are readily obtained from the graphical plots of M(H) loops of FIG. 2 from the change in slope of M(H) and is seen to steadily decrease with increase in the amount x of Cr alloying element of the CoCr ferromagnetic alloy layers.
Moreover, as is evident from FIG. 3, similar behavior is observed with CoCrPt ferromagnetic alloys, as in sandwich-type Co100xe2x88x92xxe2x88x92yCrxPty (30 xc3x85)/Ru/Co100xe2x88x92xxe2x88x92yCrxPty (30 xc3x85) structures, for x=0, 5, 10 and y=0 and 10 (again with the thickness of the Ru spacer layer adjusted to provide the greatest amount of AFC), wherein the AFC decreases with increase in the amount x of Cr and/or the amount y of Pt in the CoCrPt ferromagnetic alloys.
Accordingly, there exists a need for improved methodology for providing thermally stable, high areal recording density magnetic recording media, e.g., longitudinal media, with increased strength magnetic coupling between a pair of ferromagnetic layers separated by a non-magnetic spacer layer (such as of Ru), wherein each of the pair of ferromagnetic layers is formed of a ferromagnetic alloy composition similar to compositions conventionally employed in fabricating longitudinal magnetic recording media, which methodology can be implemented at a manufacturing cost compatible with that of conventional manufacturing technologies for forming high areal recording density magnetic recording media. There also exists a need for improved, high areal recording density magnetic media, e.g., in disk form, which media include at least one pair of magnetically coupled ferromagnetic alloy layers separated by a non-magnetic spacer layer, wherein each of the ferromagnetic layers is formed of a ferromagnetic alloy composition similar to compositions conventionally utilized in longitudinal magnetic recording media and the magnetic coupling between the layers is enhanced, leading to improved thermal stability.
The present invention, therefore, addresses and solves problems attendant upon forming high areal recording density magnetic recording media, e.g., in the form of hard disks, which media utilize magnetic coupling between spaced-apart pairs of ferromagnetic layers for enhancing thermal stability, while providing full compatibility with all aspects of conventional automated manufacturing technology. Moreover, manufacture and implementation of the present invention can be obtained at a cost comparable to that of existing technology.
An advantage of the present invention is an improved, high areal recording density magnetic recording medium having increased thermal stability.
Another advantage of the present invention is an improved, high areal recording density, longitudinal magnetic recording medium having enhanced coupling between spaced-apart magnetic layers.
Yet another advantage of the present invention is an improved method for enhancing coupling between spaced-apart magnetic layers.
Still another advantage of the present invention is an improved method for enhancing coupling between spaced-apart magnetic layers of a high areal recording density magnetic recording medium.
Additional advantages and other features of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized as particularly pointed out in the appended claims.
According to one aspect of the present invention, the foregoing and other advantages are obtained in part by a high areal recording density magnetic recording medium having improved thermal stability, comprising:
a non-magnetic substrate having at least one surface; and
a layer stack overlying the at least one surface, comprised of at least one layer pair composed of first and second superposed ferromagnetic layers spaced-apart by a magnetic coupling structure comprising a thin non-magnetic spacer layer and at least one thin ferromagnetic interface layer at at least one interface between the non-magnetic spacer layer and the first and second ferromagnetic layers;
wherein the thickness of the ferromagnetic interface layer is selected to provide enhancement of the magnetic coupling between the pair of ferromagnetic layers, thereby increasing the thermal stability of the magnetic recording medium.
According to embodiments of the present invention, the magnetic coupling structure is composed of said non-magnetic spacer layer and a ferromagnetic interface layer at one of the interfaces between the first and second ferromagnetic layers and the non-magnetic spacer layer, e.g., the ferromagnetic interface layer is at the interface between the first ferromagnetic layer and the non-magnetic spacer layer or the ferromagnetic interface layer is at the interface between the second ferromagnetic layer and the non-magnetic spacer layer.
In accordance with further embodiments of the present invention, the magnetic coupling structure is composed of the non-magnetic spacer layer and a thin ferromagnetic interface layer at each of the interfaces between the first and second ferromagnetic layers and the non-magnetic spacer layer.
According to still further embodiments of the present invention, the layer stack includes a plurality of layer pairs of first and second ferromagnetic layers, the first and second ferromagnetic layers of each pair being spaced-apart by a magnetic coupling structure comprised of a non-magnetic spacer layer and at least one thin ferromagnetic interface layer.
In accordance with particular embodiments of the present invention, the first and second ferromagnetic layers each comprise an about 10 to about 300 xc3x85 thick layer of an alloy of cobalt (Co) with at least one of platinum (Pt), chromium (Cr), boron (B), iron (Fe), tantalum (Ta), nickel (Ni), molybdenum (Mo), vanadium (V), niobium (Nb), and germanium (Ge); the non-magnetic spacer layer comprises an about 2 to about 20 xc3x85 thick layer of a non-magnetic material selected from ruthenium (Ru), Ru-based alloys, Cr, and Cr-based alloys, e.g., an about 6 to about 10 xc3x85 thick layer of Ru or a Ru-based alloy; and the at least one thin ferromagnetic interface layer comprises an about 1 monolayer thick (i.e., xcx9c1-2 xc3x85) to an about 40 xc3x85 thick layer of a ferromagnetic material having a saturation magnetization value Ms  greater than 500 emu/cc, e.g., an about 2 to about 20 xc3x85 thick layer of Co or a Co alloy with at least one of Pt, Cr, B, Fe, Ni, and Ti, wherein the Co concentration in the alloy may either be constant or varied across the thickness of the interface layer from high near the interface with the spacer layer to low near an interface with a ferromagnetic layer.
According to further embodiments of the present invention, a longitudinal magnetic recording medium comprises:
seed and underlayers between the at least one surface of the substrate and the layer stack for controlling the crystallographic texture of the at least one layer pair of first and second ferromagnetic layers;
a layer stack wherein the first and second superposed ferromagnetic layers are each comprised of an about 10 to about 300 xc3x85 thick layer of an alloy of Co with at least one of Pt, Cr, B, Fe, Ta, Ni, Mo, V, Nb, and Ge, the first and second ferromagnetic layers being spaced-apart by a magnetic coupling structure comprising a thin non-magnetic spacer layer comprised of an about 6 to about 10 xc3x85 thick layer of Ru or a Ru-based alloy and at least one thin ferromagnetic interface layer comprised of an about 2 to about 20 xc3x85 thick layer of Co or a Co alloy; and
overcoat and lubricant topcoat layers provided on an upper surface of the layer stack.
According to another aspect of the present invention, a method of forming a magnetic recording medium having improved thermal stability is provided, the method comprising steps of:
(a) forming a layer stack overlying at least one surface of a non-magnetic substrate, the layer stack including at least one layer pair composed of first and second superposed, spaced-apart ferromagnetic layers; and
(b) providing a magnetic coupling structure between the at least one pair of first and second ferromagnetic layers, the magnetic coupling structure comprising a thin non-magnetic spacer layer and at least one thin ferromagnetic interface layer at at least one interface between the non-magnetic spacer layer and the first and second ferromagnetic layers;
wherein step (b) includes a step of selecting the thickness of the thin ferromagnetic interface layer of the magnetic coupling structure to provide maximum enhancement of magnetic coupling between the pair of superposed, spaced-apart ferromagnetic layers, for increasing the thermal stability of the magnetic recording medium.
According to embodiments of the present invention, the method comprises providing the ferromagnetic interface layer at one of the interfaces between the first and second ferromagnetic layers and the non-magnetic spacer layer, e.g., at the interface between the first ferromagnetic layer and the non-magnetic spacer layer, or at the interface between the second ferromagnetic layer and the non-magnetic spacer layer.
In accordance with other embodiments of the present invention, the method comprises providing a ferromagnetic interface layer at each of the interfaces between the first and second ferromagnetic layers and the non-magnetic spacer layer.
Embodiments of the present invention include providing each of the first and second ferromagnetic layers as an about 10 to about 300 xc3x85 thick layer of an alloy of Co with at least one of Pt, Cr, B, Fe, Ta, Ni, Mo, V, Nb, and Ge; the thin non-magnetic spacer layer as an about 2 to about 20 xc3x85 thick layer of a non-magnetic material selected from Ru, Ru-based alloys, Cr, and Cr-based alloys; and the at least one thin ferromagnetic interface layer as an about 1 monolayer thick (i.e., 1-2 xc3x85) to about 40 xc3x85 thick layer of a ferromagnetic material having a saturation magnetization value Ms  greater than 500 emu/cc.
According to preferred embodiments of the invention, the method comprises providing the thin non-magnetic spacer layer as an about 6 to about 10 xc3x85 thick layer of Ru or a Ru-based alloy and the at least one thin ferromagnetic interface layer as an about 2 to about 20 xc3x85 thick layer of Co or a Co alloy, wherein the concentration of Co in the alloy is constant or varies across the thickness of the interface layer from high near the interface with the spacer layer to low near an interface with a ferromagnetic layer.
Still another aspect of the present invention is a high areal density magnetic recording medium having improved thermal stability, comprising:
at least one pair of superposed, spaced-apart ferromagnetic layers, and means for enhancing the strength of magnetic coupling between the ferromagnetic layers.
Additional advantages and aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein embodiments of the present invention are shown and described, simply by way of illustration of the best mode contemplated for practicing the present invention. As will be described, the present invention is capable of other and different embodiments, and its several details are susceptible of modification in various obvious respects, all without departing from the spirit of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as limitative.